Modify left-search formula to replace letter with number

I have attached a sample of a formula to extract the postcode sector from a full postcode address. My problem being
that when I receive the postcode address the various inputters put in a letter O instead of the number 0. The O can appear only in the 3rd, 4th 5th 6th position in the postcode and can sometimes appear twice in the same postcode.
Can someone please advise how I can modify the formula to replace the O's by number zero either by extending my existing formula or by inserting a new formula.
Many thanks
Mikey